On Bayesian routes to unit roots

This paper is a comment on P.C.B. Phillips, “To criticise the critics: an objective Bayesian analysis of stochastic trends” [Phillips (1990)]. Departing from the likelihood of an univariate autoregressive model different routes that lead to a posterior odds analysis of the unit root hypothesis are explored, where the differences in routes are due to the different choices of the prior. Improper priors like the uniform and the Jeffreys prior are less suited for Bayesian inference on a sharp null hypothesis as the unit root. A proper normal prior on the mean of the process is analyzed and empirical results using extended Nelson/Plosser data are presented.